---
title: Monitoring Low Memory When Querying
---

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The query monitoring feature prevents out-of-memory exceptions from occurring when you execute queries or create indexes.

You enable this feature when you set 
a `critical-heap-percentage` attribute for the resource-manager element
in the `cache.xml` file or by using the
`cache.getResourceManager().setCriticalHeapPercentage(float heapPercentage)`
API. When this feature is enabled and
heap memory usage exceeds
the threshold due to running a query or creating an index,
the resource manager throws an exception
and cancels the running query or index creation.

You can explicitly disable this feature by setting the system property `gemfire.cache.DISABLE_QUERY_MONITOR_FOR_LOW_MEMORY` to true.

When system memory is low,
as determined by the critical heap percentage threshold defined in 
the `cache.xml` file or in the getResourceManager API,
queries will throw a `QueryExecutionLowMemoryException`.
Any indexes that are in the process of being created will throw
an `InvalidIndexException` with the message indicating the reason.

## <a id="topic_685CED6DE7D0449DB8816E8ABC1A6E6F__section_2E9DEEC9D9C94D038543DDE03BC60B20" class="no-quick-link"></a>Partitioned Region Queries and Low Memory

Partitioned region queries are likely causes for out-of-memory exceptions. If query monitoring is enabled, partitioned region queries drop or ignore results that are being gathered by other servers if the executing server is low in memory.

Query-monitoring does not address a scenario in which a low-level collection is expanded while the partitioned region query is gathering results. For example, if a row is added and then causes a Java level collection or array to expand, it is possible to then encounter an out-of-memory exception. This scenario is rare and is only possible if the collection size itself expands before a low memory condition is met and then expands beyond the remaining available memory. As a workaround, in the event that you encounter this situation, you may be able to tune the system by additionally lowering the `critical-heap-percentage`.


